The present invention relates to an optical sensor.
Optical sensors for determining the concentration of a gas such as carbon dioxide in the air are used, among other things, in fire alarms. Their function is based on the fact that a sensitive layer of the sensor changes color reversibly upon contact with the gas to be determined. This change of color is recorded by a detector, and when a predetermined minimum concentration is exceeded, an alarm is triggered.
In U.S. Pat. No. 5,480,611 an optical sensor for determining carbon dioxide content of the air for medical applications is described, which has a layer sensitive to carbon dioxide applied to a substrate. This includes a fluorescing pH indicator, whose fluorescence changes as a function of the carbon dioxide concentration in the surrounding solution. In addition, the sensitive layer contains quaternary ammonium salts which provide for a basic environment in the sensitive layer and improve the CO2 absorbing capacity of the layer. Tetraoctylammonium hydroxide is especially preferred for this purpose. However, during the production process of the sensor, this quaternary ammonium compound demonstrates poor miscibility with the polymer solution of which the sensitive layer is made.
It is an object of the present invention to produce an optical sensor for determining a gas which has a short response time and a high sensitivity, which is produced simply and therefore cost-effectively.
The optical sensor according to the present invention has the advantage that it has a very short response time and very high sensitivity with respect to a gas to be determined. This is achieved by adding to the sensitive layer of the sensor a phosphorus- or a nitrogen-containing base having numerous and/or long chain alkyl groups. Such bases demonstrate a sufficiently high basicity and very good miscibility with organic polymers. This simplifies the production of the sensitive layer of the sensor.
Thus, a phosphorus-containing base, preferably an alkylated phosphazene is used, because it most markedly shows the above-named favorable characteristics. Especially advantageous is the use of the phosphazene P4-t-octyl, having the empirical formula [(CH3)2N)3Pxe2x95x90Nxe2x88x92]3Pxe2x95x90N(C8H17).
According to the present invention, as a nitrogen-containing base, a quaternary ammonium salt is suitable, having long-chain alkyl groups which have more than 10 carbon atoms. In this regard, bases of the general composition [(CxH2x+1)aNR4xe2x88x92a)]+OHxe2x88x92 are especially advantageous, where R stands for an alkyl group, x is a number greater than 10 and a is a number between 1 and 4.
In addition, the sensitive layer of the sensor is advantageously made as a polymer matrix, and here the use of polydimethylsiloxane as the polymer leads to a great sensitivity of the layer on account of its characteristic gas permeability.